Seismic expression of active extensional faults in northern Umbria (Central Italy)

This paper deals with the geometry and kinematics of the active normal faul ts in northern Umbria, and their relationship with the seismicity observed in the area. In particular, we illustrate the contribution of:seismic refle ction data (a network of seismic profiles, NNW-SSE and WSW-ENE trending) in constraining at depth the geometry of the different active fault systems a nd their reciprocal spatial relationships. The main normal fault in the are a is the Alto Tiberina fault, NNW trending and ENE dipping, producing a dis placement of about 5 km, and generating a continental basin (Val Tiberina b asin), infilled by up to 1500 m with Upper Pliocene-Quaternary deposits. Th e fault has a staircase trajectory, and can be traced on the seismic profil es to a depth of about 13 km. A set of WSW-dipping,antithetic faults can be recognised on the profiles, the most important of which is the Gubbio faul t, bordering an extensional Quaternary basin and interpreted as,an active f ault based on geological, geomorphologic and seismological evidence. The ep icentral distribution of the main historical earthquakes is strictly parall el to the general trend of the normal faults. The focal mechanisms of the m ajor earthquakes show a strong similarity with the attitude of the extensio nal faults, mapped at the surface and recognised on the seismic profiles. T hese observations demonstrate the connection between seismicity in the area and the activity of the normal faults. Moreover, the distribution of the i nstrumental seismicity suggests the activity of the Alto Tiberina fault as the basal detachment for the extensional tectonics of the area. Finally, th e action of the Alto Tiberina:fault was simulated using two dimensional fin ite element modelling: a close correspondence between the concentration of shear stresses in the model and the distribution of the present earthquakes was obtained. (C) 1999 Elsevier Science Ltd. All rights reserved.